Light-emitting elements such as organic EL lighting devices have recently attracted attention as eco-friendly green devices. The organic EL lighting devices are characterized by 1) lower power consumption than incandescent lamps; 2) thin profile and light weight; and 3) flexibility. The organic EL lighting devices are now being developed to achieve the features 2) and 3). In this respect, glass substrates conventionally used in flat panel displays (FPD) or the like cannot achieve the features 2) and 3).
In this regard, researches have been conducted on a substrate as a support (hereinafter, referred to as “supporting substrate”) for organic EL lighting devices, and ultra-thin glass plates, resin films, metal foils or the like have been proposed as candidate supporting substrates. The ultra-thin glass is superior in heat resistance, barrier performance, and optical transparency and has good flexibility, but is somewhat inferior in handleability and has low thermal conductivity and high material cost. The resin film is superior in handleability and flexibility and has low material cost and good optical transparency, but is inferior in heat resistance and barrier performance and has low thermal conductivity.
In contrast, the metal foil has excellent characteristics such as superior heat resistance, barrier performance, handleability, and thermal conductivity, good flexibility, and low material cost, except for absence of optical transparency. In particular, a typical flexible glass or film has a significantly low thermal conductivity of 1 W/m ° C. or lower, while a copper foil has a significantly high thermal conductivity of about 400 W/m ° C.
PTL 1 (JP2009-152113A) discloses formation of an organic layer on a surface of a metal substrate that is smoothed by polishing and/or plating in order to achieve a light-emitting element including the metal substrate. PTL 2 (JP2008-243772A) discloses formation of an organic EL element on a smooth surface of a nickel plating layer that is formed on a metal substrate without polishing or the like. Aside from these disclosures, a photoelectric element including a metal substrate is proposed; for example, PTL 3 (JP2011-222819A) discloses a solar cell with a thin organic electromotive layer provided on a smoothed metal substrate. In these techniques, however, the smoothing of the metal substrate is an important challenge for prevention of a short circuit between the electrodes. For addressing this challenge, PTL 4 (WO2011/152091) and PTL 5 (WO2011/152092) disclose metal foil having an ultra-smooth surface having an extremely low arithmetic mean roughness Ra of 10.0 nm or less that functions as both supporting substrates and electrodes.
Furthermore, an electrolytic copper foil for a lithium-ion secondary battery is known, which has a surface roughness Rz of 0.8 to 2.8 μm on both surfaces and a 0.2% proof stress of 250 N/mm2 or more after heat treatment at 200 to 400° C. (see PTL 6 (JP2012-151106A)).